Retirement Plans and Other Postretirement Benefits - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended
	Sep. 30, 2014	Jul. 31, 2012	Mar. 31, 2016	Mar. 31, 2015
Regulatory Assets
Regulatory asset amortization period		3 years
Amortization of regulatory asset				$ 2,000,000
Pension Benefits
Contributions
Voluntary employer contributions to pension plan			$ 60,000,000
Minimum employer contributions for the next three years			0
Maximum employer contributions for the next two years (up to)			300,000,000
Other Benefits
Other Postretirement Benefit Plan Remeasurement
Other postretirement plan benefit remeasurement, amount seeking approval to move to separate account to Pay Union employee medical costs	$ 140,000,000
Contributions
2016 (up to)			1,000,000
2017 (up to)			1,000,000
2015			$ 1,000,000